program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  tpoint = record
    x, y : extended;
  end;

var
  point : array[1..1000000] of tpoint;
  i, n : integer;
  ans : extended;

function cross(p1, p2 : tpoint) : extended;
begin
  result := p1.x * p2.y - p2.x * p1.y;
end;

begin
  reset(input, 'input.txt');
  rewrite(output, 'output.txt');

  read(n);
  for i := 1 to n do
    read(point[i].x, point[i].y);
  point[n + 1] := point[1];

  ans := 0;
  for i := 1 to n do
    ans := ans + cross(point[i], point[i + 1]);

  writeln(abs(ans / 2):0:8);

end.
